Not Applicable
Not Applicable
The present invention relates in general to alignment structure and method for a multiple-field camera, and more particular, to alignment structure and method to align the multiple-field camera using a single spot light source.
Image registration is a fundamental task required in many applications of image processing which involves taking two or more images and aligning them so as to either eliminate differences between them or highlight the salient differences for the purpose of study. The former situation occurs within such areas as target matching and the latter when producing depth information from stereo pairs or mapping changes over time. Currently, there are essentially four different types of registration, including multi-modal registration, template registration, viewpoint registration and temporal registration. In multi-modal registration, images have been taken by different types of sensors. In template registration, a referenced image is to be found inside a larger image. This is useful for locating a specific feature on a map. The viewpoint registration images the same object with two similar sensors at the same moment from different positions, while temporal registration photographs the same object from the same viewpoint at different times. Single element multiple field cameras have been designed to capture well aligned temporally registered images. To obtain sub-pixel accuracy of any of the above registrations between optical fields, well-registered and stable camera configurations are required.
An integrated lens has been used in the single element multiple field cameras. In this design, the lens position is constantly referenced to the same mechanical plane referred as a focal plane. This minimizes allowable movements critical to maintaining alignment. However, since the manufacture of the focal plane leaves position errors of greater than {fraction (1/10)} pixel of the camera to external physical features (typically 2 microns), a mechanical alignment to physical attributes is not possible unless pixels are visible.
In U.S. Pat. No. 5,479,015, a multi-image detector assembly has been disclosed. The multi-image detector assembly allows image detection occurring continuously and simultaneously so as to provide a spatially and temporally correlated set of separate images utilizing a single focal plane. However, alignment or correction of position of the focal plane relative to the lens is not addressed.
The present invention provides an alignment structure and an alignment method which generate a plurality of spot images from the same light source and aligns individual spot images between different fields independently without affecting the alignment of other spot images. The camera includes a plurality of focusing members referenced to a common focal plane array of a plurality of detectors. The alignment structure comprises a light source, a collimator and a multiple-beam generator. The light source is operative to generate a light beam suitable for spectral characteristics of the camera, and the collimator collimates the light beam into a collimated light spot with a predetermined dimension. The multiple-beam generator then splits the light beam into a plurality of components converging on the camera at angles within field of view of the camera. In one embodiment, the predetermined dimension is larger than one pixel and smaller than three pixels of the camera to result in a spot image with energy significant in the region of 5xc3x975 pixels of the focal plane.
The multiple-beam generator further comprises a platform, a central aperture, a plurality of peripheral apertures, a plurality of first mirrors and a plurality of second mirrors. The central aperture perforates through a center of the platform by a dimension smaller than the predetermined dimension of the collimated light spot. Therefore, by aligning the centroid of the collimated light spot with the central aperture, only a central portion of the collimated light beam propagates through the platform as a central component. The peripheral apertures are formed by perforating the platform about the central aperture. The first mirrors are arranged on the platform to split the edge portion of the collimated light spot into a plurality of peripheral components and reflect the peripheral components onto the second mirrors, while the second mirrors are positioned and oriented to reflect the peripheral components to propagate through the apertures and converge at the focusing members. The first and second mirrors are further adjusted to converge the peripheral components at the camera at angles approaching field of view of the camera.
The alignment structure further comprises a kinematic stage for disposing the focusing members of the camera thereon. The kinetic stage is preferably operative to translate and rotate the focusing members with 6 degrees of freedom. Therefore, the orientations and positions of focusing members of the camera can be adjusted to optimize the spot images captured on the focal plane from the central and peripheral components.
The present invention further provides a single-element multiple-field camera system comprising a camera and an alignment structure. The camera comprises a plurality of focusing members and a common focal plane array of a plurality of detectors for the focusing members. The alignment structure comprises a light source operative to generate a light beam suitable for spectral characteristics of the camera, a collimator disposed along an optical path of the light beam generated by the light source, a multiple-beam generator disposed along an optical path of the light beam propagating through the collimator, and a kinematic stage for translating and rotating the focusing members relative to the focal plane array.
The focusing members are integrated into a single slab of material, and the material includes silicon. The collimator is operative to collimate the light beam generated by the light source into a collimated spot light with a dimension larger than one detector and smaller than three detectors of the focal plane array. The multiple-beam generator comprises a platform, a central aperture, a plurality of first mirrors, a plurality of second mirrors, and a plurality of peripheral apertures. The platform is disposed perpendicular to propagation of the collimated spot light. The central aperture is perforated through the platform and aligned with centroid of the collimated spot light. The first mirrors are mounted on the platform and are positioned and oriented to split an edge portion of the collimated spot light into a plurality of peripheral components. The peripheral apertures are perforated through the platform around the central aperture. The second mirrors are mounted on the platform and positioned and oriented to reflect the peripheral components to propagate through the peripheral apertures and converge at the camera
By the above alignment structure, a central spot image and a plurality of peripheral spot images around the central spot image are formed on the focal plane array for each focusing member. The camera can thus be aligned by translating and rotating the focusing members along various directions to overlap the central and peripheral spot images formed for different focusing members.
In one embodiment, the multiple-beam generator includes four first mirrors, four second mirrors and four peripheral apertures arranged to split the edge portion of the light source into four peripheral components; and consequently, four peripheral spot images are formed by each focusing member.
The present invention further provides an alignment method for a camera with a multiple-element lens and a focal plane array of pixels. The method comprises the following steps. A light beam suitable for spectral characteristics of the camera is generated. The light beam is collimated into a light spot with a predetermined dimension. The light spot is split into one central component and a plurality of peripheral components about the central component, such that one central spot image and a plurality of peripheral spot images are formed on the focal plane array for each field of the camera. The multiple-element lens is then translated and rotated along various directions to optimize spot images of the central and peripheral components captured on the focal plane array.
In the above method, the light beam is preferably collimated into the light spot with a dimension larger than one pixel and smaller than three pixels of the camera. The light spot is preferably split into the central and peripheral components converging at the multiple-element lens with angles within field of view of the camera. The step of translating and rotating the multiple-element lens comprises the following steps (a) to (g). In step (a), the multiple-element lens is translated along a first direction for coarsely focusing the central and peripheral spot images. In step (b), the multiple-element lens is rotated about the first direction for minimizing distance of the central spot images between the fields. In step (c), the multiple-element lens is rotated along a second direction and a third direction perpendicular to the first direction for offsetting difference of origins between the fields. In step (d), the multiple-element lens is rotated about the first direction for refining alignment of central spot images between the fields. In step (e), the multiple-element lens is rotated about the second direction to minimize distance of the peripheral spot images arranged along the second direction between the fields. In step (f), the multiple-element lens is rotated about the third direction to minimize distance of the peripheral spot images arranged along the third direction between the fields. In step (g), the multiple-element lens is translated along the first direction for further focusing the central and peripheral spot images.
The alignment method as discussed above further comprising repeating steps (d) to (f) until the distances of the central components and the peripheral components between the fields are within a predetermined tolerance, and repeating step (g) until the spot images are focused within a predetermined tolerance. In one embodiment, the predetermined tolerance of the distance is {fraction (1/10)} pixel of the focal plane.